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cmd/compile: use cheaper implementation of oneBit

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This is the second attempt. The first attempt was CL 229127,
which got rolled back by CL 229177, because it caused
an infinite loop during compilation on some platforms.
I didn't notice that the trybots hadn't completed when I submitted; mea culpa.

The bug was that we were checking x&(x-1)==0, which is also true of 0,
which does not have exactly one bit set.
This caused an infinite rewrite rule loop.

Updates #38547

file    before    after     Δ       %
compile 19678112  19669808  -8304   -0.042%
total   113143160 113134856 -8304   -0.007%

Change-Id: I417a4f806e1ba61277e31bab2e57dd3f1ac7e835
Reviewed-on: https://go-review.googlesource.com/c/go/+/229197
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Munday <mike.munday@ibm.com>
This commit is contained in:
Josh Bleecher Snyder 2020-04-20 15:40:38 -07:00
parent 0eb694e9c2
commit 4974ac6874
1 changed files with 5 additions and 5 deletions
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10
src/cmd/compile/internal/ssa/rewrite.go
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@ -397,11 +397,11 @@ func ntz32(x int32) int { return bits.TrailingZeros32(uint32(x)) }
func ntz16(x int16) int { return bits.TrailingZeros16(uint16(x)) }
func ntz8(x int8) int { return bits.TrailingZeros8(uint8(x)) }
func oneBit(x int64) bool { return bits.OnesCount64(uint64(x)) == 1 }
func oneBit8(x int8) bool { return bits.OnesCount8(uint8(x)) == 1 }
func oneBit16(x int16) bool { return bits.OnesCount16(uint16(x)) == 1 }
func oneBit32(x int32) bool { return bits.OnesCount32(uint32(x)) == 1 }
func oneBit64(x int64) bool { return bits.OnesCount64(uint64(x)) == 1 }
func oneBit(x int64) bool { return x&(x-1) == 0 && x != 0 }
func oneBit8(x int8) bool { return x&(x-1) == 0 && x != 0 }
func oneBit16(x int16) bool { return x&(x-1) == 0 && x != 0 }
func oneBit32(x int32) bool { return x&(x-1) == 0 && x != 0 }
func oneBit64(x int64) bool { return x&(x-1) == 0 && x != 0 }
// nlo returns the number of leading ones.
func nlo(x int64) int64 {